In general, the present invention is situated in the context of precaution and/or protection means for detecting hazardous airborne particles. The growing concentration of different types of aerosols in the atmosphere becomes more and more an important issue. Their specific influence on the climate change and air quality is still an open question. More than that, recently new threats like bio-terrorism that employs biological particles as an arm of massive destruction have appeared. Reliable and cost-effective aerosol detectors, fast and with a high discrimination power, are still highly desirable in the market.
Particular species of aerosol particles, like pollens and spores, have a large impact on human health. Some of them are responsible for health problems like allergies affecting, according to statistical analysis, about 20% of the European population. The size of potentially hazardous airborne particles spreads over a range situated approximately between parts of a micrometer and some hundred micrometers.
A variety of different measurement devices and methods for a detection of airborne particles are known, which are mainly based on light scattering measurements for estimating the size of particles contained in an aerosol and, in some cases, detection of the position of the particle at a given moment.
A first known, simplest and in industry still most applied, measurement device and method for this purpose is based on one laser emitting a spatially shaped laser light beam for obtaining an elliptical laser light spot in a measurement region, where an air flow from a nozzle crosses the laser beam, and one photodetector for detecting scattered light from a particle contained in the air flow and crossing the laser beam is provided. An output signal from the photodetector is then used to calculate the particle size.
Another known approach is based on the use of one laser in combination with two photodetectors for acquiring, in different angular orientation with respect to the laser beam, scattering signals from a particle crossing the laser beam.
Because of signal acquisition in two different light scattering angles, this approach allows for a more precise determination of a particle size thanks to simultaneous acquisition of two scattering patterns than the first approach described above. However, this approach is not adequate for a sufficient determination of the exact location of a detected particle.
A further known approach is based on the use of two lasers emitting laser light at different wavelengths in combination with two photodetectors provided with different spectral filters adapted to the two different laser emission wavelengths for acquiring scattered laser light from a particle crossing the laser beams, selectively for the emission wavelengths of the two lasers and central transmission wavelengths of optical band pass filters provided to the photodetectors designed for selective detection of laser light from an assigned laser. However, this approach suffers from limitations in particle size measurement because, for both photodetectors, only scattered light from one laser at a detection wavelength assigned to the related light-emitting laser is detected. Consequently, also this known approach is not adequate for solving imprecisions of particle size determination, particularly because of a potentially chromatically based error, and a correct determination of an actual particle location is also impaired by the inherent disadvantages of this approach.
The solutions according to prior art therefore all suffer from an insufficient precision of a determination of size and location of airborne particles.